I think 6-8 females is the norm for a heavily planted (you want lots of hiding places) 10 gallon (Absolutely no Males, as the girls would most likely pick him apart..) Hopefully someone more knowledgeable will have a more in depth/definitive answer for you. In regards to adding other fish, personally, I would not add other fish, especially, if you're going with a 10 gallon...

No less than 5 and no more than 8. Less than 5 will result in too much fighting and more than 8 will be overstocking. However, even with 8 you are pushing it. So, maybe invest in a larger filter to handle the extra bioload.

Definitely no males in the tank. I would say not even if it is divided, as females become "eggy" in the presence of a male. Being "eggy" for a long period of time would not be healthy. I would also imagine that the presence of a male would cause a lot more fighting among the females.

I have heard of people putting as many as ten girls in a ten gallon, but that was with a higher rated filter and many live plants. I have six girls in mine currently and two more in quarantine. This will be my final number, although I do plan on adding a snail at some point. BUT I am running an Aqueon Quietflow20, so I have twice the filtration I would typically need for a ten gallon. I also have live plants and keep up with my weekly 50% water changes.

Sororities are great fun, but remember to keep it to all girls! The male would probably be killed, and even if he's not, he'd promote a LOT more aggression. Also, make sure you have a backup plan in case any of your girls aren't suited to sorority life. Some fish just can't handle it.

I would do 5 in a 10G. Be careful if you get young ones as they can be mislabled males.
And add lots and lots of silk plants that reach all levels of the tank. They less they see each other, the less they can fight